1. Field of the Invention
The present invention relates to an exhaust gas heat exchanger, for performing heat exchange between exhaust gases produced when fuel is burnt in an engine and a cooling fluid such as water and, more particularly, to an exhaust gas heat exchanger for cooling exhaust gases for an EGR (exhaust gas recirculation) system (hereinafter referred to as an “EGR-gas heat exchanger”).
2. Description of the Related Art
As a conventional EGR-gas heat exchanger, an EGR-gas heat exchanger is described, for example, in Japanese Unexamined Patent Publication No. 2001-33187 (Kokai). The heat exchanger is constituted by a plurality of stacked tubes which are received in the interior of a tank. The tank is closed with end plates (core plates) and the tubes are secured to the core plates. Connected to the tank are an coolant inlet pipe and an coolant outlet pipe, whereby coolant flows into the tank to remove heat from exhaust gases which pass through the tubes.
It is generally known, as one of means for improving the heat exchange capacity of a heat exchanger, to provide inner fins in tubes of the heat exchanger. Known generally, as a method for producing such a tube, is a method comprising the steps of, for example, inserting an inner fin in a welded tube, bringing the tube into close contact with the inner fin by applying an external force to the tube, and brazing the inner fin to the tube.
Incidentally, with an EGR-gas heat exchanger, an Ni system brazing material is used to braze respective members in order to prevent corrosion caused by condensate produced when exhaust gases are cooled. In general, a brazing material in a paste form is used as the Ni system brazing material and is thinly applied to portions to be joined together.
Thus, in the event that the aforesaid production method is used, in which the inner fins are inserted into the tubes, the applied brazing material is stripped off when the inner fin is inserted, leading to a possibility that sufficient brazing material cannot be provided between the tube and the inner fin.
To cope with this problem, the inventor, et al. produced, by way of a trial, and studied a tube 1, as shown in FIG. 8, which is constituted by a pair of plates 2, 3 adapted to fit with each other in such a manner as to put an inner fin 4 between the plate 2 and the plate 3, as a tube for an EGR-gas heat exchanger in which an inner fin is accommodated.
Since the tube shown in FIG. 8 is constructed such that the pair of plates 2, 3 fit with each other in such a manner as to put the inner fin 4 between the plate 2 and the plate 3, while the aforesaid stripping off of the brazing material due to assembling the inner fin 4 to the tube 1 can be prevented, a difference in level corresponding to the thickness of the outer plate 2 is produced on the external wall surface of the tube 1. It has been made clear that due to this, when the tube 1 is passed through a core plate (not shown) a gap corresponding to the difference in level is produced between an edge of an opening in the core plate and the tube 1 and hence a failure in brazing is caused. Then, when a failure in brazing occurs between the core plate and the tube 1 there occurs a risk that there is caused a leakage between an exhaust gas passage and a coolant passage which are partitioned by the core plate.